Reuse existing Kubernetes manifests in K8s charms

[knkski]

Some feedback from upstream projects where we've added charms is that it would be nice to be able to automatically generated the pod spec from existing manifests:

kubeflow/katib#1465 (comment)

[pengale]

I believe that we have considered this, and decided against it for now.

We want to avoid the situation where we have a lot of charms that are just a little bit of boilerplate around an existing pod spec.

It's perfectly valid to take an existing spec, and think carefully about how to express it as a charm. But (for now), we aren't going to provide tooling to do this automatically.